1. Field of the Invention
This invention relates to a fuel feed system. More particularly, it relates to an electronic fuel feed system which is the most suitable for controlling the air fuel ratio of a mixture to be sucked into the engine of an automobile.
2. Description of the Prior Art
It has been known to use a swirl-type air flow meter or a vortex-type air flow meter for the measurement of the quantity of suction air which is sucked into an automobile engine. The air flow meter is such that vortices at a frequency proportional to the velocity of flow are generated and that the generated vortices are electrically detected as pulses. Since the vortex frequency is very low, a method has been adopted wherein an analog voltage corresponding to the number of vortices is produced and wherein the fuel injection time is determined from the discharge time of the voltage. Such a method is disclosed in, for example, U.S. Pat. No. 3,818,877 granted to Ford Motor Company. Digital circuits are more easily put into the form of integrated circuits, and are more immune to noise. For such reasons, it is desirable to digitally control the quantity of fuel supply. Accordingly, there has been developed a method wherein the average flow rate of air sucked into the engine is evaluated from the inverse number of the vortex period and is divided by the engine revolutions, whereby the instantaneous flow rate of air per combustion cycle is detected so as to control the fuel injection time.
Letting N denote the engine revolutions and Qa denote the flow rate of air sucked into the engine, the instantaneous flow rate of air Q sucked into the engine per combustion cycle is indicated by the following equation: EQU Q.varies.Qa/N (1)
The output frequency of the air vibration flow meter is proportional to the flow rate of air sucked into the engine. Therefore, letting f denote the frequency and .tau. denote the period, the following equation holds: EQU Q.varies.f/N (2)
or EQU Q.varies.1/.tau..multidot.N (3)
The instantaneous flow rate of air Q is measured with Eq. (2) or Eq. (3). In the measuring method employing Eq. (2), the number of vortices per unit engine revolution, in other words, the number of vortices per combustion cycle is small, and the measuring accuracy is insufficient. On the other hand, in the measuring method of Eq. (3) exploiting the inverse number of the vortex period .tau., the varying range of .tau. is as very wide as 50 times. This leads to the disadvantage that, when the operations are conducted by the use of a digital control device (for example, micro-computer), the number of bits of the digital control device must be made large in order to secure an operating accuracy (because the inverse number of the vortex period needs to be calculated).